@simon I think https://caniuse.com/mdn-http_headers_set-cookie_samesite_lax_default answers that question? I haven't checked that it's correct, but generally it'd be nice for `caniuse` to answer questions about how completely each browser implements consensus and proposed standards.
@jyasskin sadly that doesn’t cover the deeper issue of what happens if you send set-cookie without a SameSite attribute at all - or weird undocumented edge-cases like what changes if a Safari user turns on “Prevent Cross-Site Tracking”